Zed Head

Google Maps has a feature that lets you look back at their previous images. Wonder how far back it might go.

Another one. Same video.

Not a movie or TV but a car show video. Couple of Z's. Might have posted it before.

Are you using the word "ground" correctly? Or understanding how the ignition system works? No offense. It sounds like you're saying that the ECU is not picking up the distributor trigger. Is it the factory variable reluctance (VR) trigger? What is under the distributor cap? Stock parts or aftermarket parts? A picture would be good. The VR trigger does not use a ground it uses two wires that transmit an alternating current wave form. It's not uncommon to have triggering problems with the aftermarket engine management systems or ignition systems, like MSD's. Some trigger systems are set up for Hall effect triggers as an option. People get them mixed up sometimes. Might be worthwhile to show your wiring scheme. The distributor ground might not be important in your case, although I think that it migth serve the purpose of dissipating static charge or stray spark plug wire spark. It's definitely important with a points system.

You could even draw on SteveJ's picture. Do something. There's a lot of different mushroom shapes out there. Might be that the diaphragm has blown and since there's no fuel passing through the pump it lets oil leak out. When fuel is flowing sometimes they leak fuel in to the crankcase.

It shouldn't really matter. Unless you screwed and unscrewed the nuts a few time while you were working on it and the studs backed out. Might be that you didn't have full thread engagement, or maybe the block threads are damaged. Since you have the head off it's probably a good time to really look at how well the studs engage the head. Just more thoughts. The ARP system is smart but the old school head bolts seem to work well on the Datsun engines. Good luck.

Just offering a different view. ARP's instructions are not the clearest but it looks like they want 60 ft-lbs if you use their lubricant. Did you use their stuff? https://tech.arp-bolts.com/instructions/202-4202.pdf

Does your theory fit? The nuts are still torqued on to the studs, pulling up on the threads, providing clamping force. Doesn't really matter if the studs aren't seated all the way to the bottom of the hole. Has the head been resurfaced? Is the deck flat? Seems more likely that the problem has a different source.

The video had some tips and tricks. Obviously, you are well past this step. Don't forget to use good break-in oil on the first engine start. The cam lobes are sensitive.

The OP is in Austin. I've read about the N42 heads over there. They were still running carburetors right?

.040" over flat tops. https://www.ebay.com/itm/165387446201?fits=Model%3A280ZX|Make%3ADatsun&epid=75271992&_trkparms=ispr%3D1&hash=item2681dc27b9:g:8OMAAOSwvkJiMhTi&amdata=enc%3AAQAGAAABIKiiCdDNms8s1aN21U%2Bmm7wiO1Yzt1v6lTAirumB82Fef3JCgqY6kRipL4DKdXgD0FAq7G4lB4Z1PC9drVf527H8gPvzl6v6C%2BMUGWburvcIMlBPo59ZzQ4aq61o5WqQa%2Fw0Qs324QvLl4dptZyuitjGtNYhI3m2nNzBGdSccVhTMZ3fCXLQekspzg9XfkQ3on4sShcbRXe9trCxUy%2BMgjbVkA9czun4vcV%2BD%2FY1rSRju4gT8T50LkSkcSpHOoeP3e9KYDMOU%2FrQwQrvlIkqIH6V9JHYb54tFE5ExVHvU41Ss5S0qVSH6hJioh0OZsJpfCF4T0CARtU9rFKWm3pDVSHvfCN0Mf8zaWbTxWukvgCJsSSTApzPTx79KySUALI%2BFA%3D%3D|tkp%3ABFBMutDWz_Zf

Here are standard pistons for what you have. https://www.ebay.com/itm/172502490849?fits=Make%3ADatsun&epid=77048862&hash=item2829f31ee1:g:FxEAAOSw9KNiFsAL

1980 (maybe 1981) to 1983 were flat tops. That was the range of the P79 heads, approximately. 1979 was definitely the N47 head and dished pistons. Check your information source, there is a lot of bad information out there. eBay shows that those are dished. So they would be like your stock pistons. If the picture is correct. Those are turbo pistons though so the top ring is set farther down the piston, I think, to protect it from excess heat. They would work but not be exactly the same. https://www.ebay.com/itm/283805353930 eBay has a bunch but the seller sites are hard to search.

I was serious. The exhaust pipe hides the other big bolt head.

I thought that it was for looks.

ZX pistons are flat top pistons and your 75 pistons are dished. But many people use the flat tops to raise the compression ratio for a bit more power. I think it would end up about 10:1. So yes they would work, you'll just end up with a higher CR.

You can have more than one ground point. There is a ground wire from the alternator body to the harness which must be attached to battery negative. And the alternator body is attached to the engine block through its mounting bolts. And battery negative is attached to the body grounds. Plus the battery negative is attached to the engine block through the starter mounting bolt. Many distributors use the mounting screws as grounds through the distributor body, but also have a separate grounding wire to the body. You can learn a lot with a meter. Posting before I look at SteveJ's reply...

Have you studied the factory curves? Vacuum advance disappears pretty quickly, and it's only vacuum (pressure based). Does the software allow you to change units? It will make things easier. There are conversion charts. I'd match a factory setup and then tune from there. https://www.vacuum-guide.com/english/equipment/conversion_pascal_bar.htm https://www.engineeringtoolbox.com/vacuum-converter-d_460.html

Can you point that out in the text? Maybe you read in to it subconsciously, with "driveshaft angularity"? I can't find anything specific about getting noise from a top mount. Quite a few people running a top mount only. It works well. Although I did make mine to match driveshaft angularity. RTz built his for a V8 application, originally, so changed the angle slightly. p.s. they call it a mount but it is actually a strap replacement when used with a snubber. Pedantic... https://www.technoversions.com/DiffMount.html

I had the clunk and made my own mount, based on the RTz design. I am pretty certain that the noise comes from the diff nose rising up then dropping back down on to the crossmember. The clunk is when it hits. There is a video or two out there showing that. So, the use of a snubber on top is really just stopping the initial rise. It works, just pointing out the why. One nice thing about hanging the diff nose from the top is that you can mess around with your rear suspension without having to deal with the hanging diff nose. But, really, a new diff mount will probably get rid of the clunk. It's really hard to tell that the mount rubber is worn unless you stick a pry bar in there and lever it up, with force. It is surprising how far it will move. I made a new mount because I bought the car for something to work on, a reason to be in the garage. It was a challenge. But today I might just buy a new diff mount if it was available.

I found some yellow ones in Australia. https://www.ebay.com.au/b/Coil-Springs-for-Nissan-Datsun-240-Z/33582/bn_7019833571 and Summit, but they aren't specific on vehicle. https://www.summitracing.com/parts/afc-25225-3?seid=srese1&gclid=CjwKCAjwoduRBhA4EiwACL5RP1FKAFgrHQZEuWSVT8JZyqMnHkertyDdfGfUFJhOg00yDEjZ945UjxoCWMgQAvD_BwE

Here's a Hybridz link with a youtube video. The guys that posted in it are some of the smart engine building guys. Might give you some ideas and the video has good images of the marks and the links. He has dark links. https://forums.hybridz.org/topic/129303-matching-cam-crank-timing/ Time-stamped the video.

There's probably a cam lobe at a certain point when things are right that you could use instead of the marks. Just about to get on the ramp or straight up, or something.

Everything seems pretty well installed, but it's just hard to tell if the cam is oriented correctly. Weird that the aftermarket companies have removed all of the marks needed to ensure proper installation. Is there a mark on the lower sprocket? I looked at the images on Rockauto and couldn't tell. They might be showing the backside. Edit - I think I see it now.